This invention relates to acoustical flowmeter systems and is particularly directed to an improvement in the acoustical flowmeters of the type described and claimed in the U.S. Pat. No. 4,003,252 entitled "Acoustical Wave Flowmeter" by E. J. DeWath which issued Jan. 18, 1977 and in the flowmeter system of type described and claimed in the U.S. Pat. No. 4,164,865 entitled "Acoustical Wave Flowmeter" by L. G. Hall and R. S. Loveland which issued Aug. 21, 1979.
The invention of DeWath was directed to a flowmeter with two spaced apart transducer elements and unobstructed tubular wall thereby eliminating all impediments to the flowpath of the fluid where debris might collect. The advantages of such a configuration are fully set forth in the DeWath patent. To measure the flow of a selected fluid in DeWath flowmeter, however, a calibration for that selected fluid was required, but, if the flow of a different fluid was to be measured, a re-calibration was required since the DeWath flowmeter was not responsive to changes in fluid species or densities.
The Hall and Loveland invention improved the DeWath flowmeter by providing a flowmeter that measured the flow accurately regardless of changes in fluid composition or temperature and by providing a flowmeter with a means of determining a change in the velocity of sound in the fluid being measured.
In order to accomplish this, the Hall and Loveland acoustical wave flowmeter system like the DeWath flowmeter had two spaced apart transducer elements in the wall of the flowmeter conduit (sometimes called a cavity) to produce ultrasonic acoustic compressions at selected frequencies in the fluid within the cavity. The transducer elements were alternately switched into a transmit and a receive mode to generate upstream and downstream transmitted and received signals with an automatic means to adjust the transmitted frequencies to compensate for changes in velocity of the acoustic compressions in the fluid caused by changes in fluid composition or temperature. The electronic circuitry involved in the Hall and Loveland flowmeter included means for measuring and storing signals representing the phase difference between the transmitted transducer element signals producing the acoustical compressions and the signals produced by the receiving transducer element in response to the received acoustic compressions during each of two successive transmit/receive cycles. Circuit means were provided to determine the difference between the signals representing the two successive phase differences with the sign of the difference corresponding to the direction of the fluid flow and the magnitude of the difference corresponding to the rate of flow through the flowmeter. Circuit means were also provided to add the two successive phase difference signals together to obtain a signal proportional to the velocity of sound in the flowmeter representing fluid density so that any change in the signal indicated a change in the composition of the fluid in the meter.
It is to be noted, before going further, that the term "transducer" has been heretofore used in two contexts. The two spaced-apart "transducers" which produce the acoustical waves in response to applied signals and which produce a signal in response to the acoustical waves are, in fact, "transducers" and the flowmeter conduit containing the two spaced apart "transducers" and through which the fluid flows is, in fact, a transducer and also sometimes called a "transducer" or a "transducer assembly" or a "meter". Now in order to distinguish the "transducers", the two spaced apart "transducers" will be called "transducer elements" and the conduit containing the "transducer elements" will continue to be called the "transducer" or "transducer assembly" or "meter",
Now continuing on, the circuitry of the Hall and Loveland flowmeter system has undergone a number of improvements, for example, by the invention in U.S. Pat. No. 4,345,479, supra, the clock system has been made synchronous within the system for generation of precise timing signals for the control of the system circuitry, the measurement of the energy at a certain time to produce an accurate phase measurement was improved by the invention in the U.S. application entitled "Flowmeter Systems for Ultrasonic Energy Improvement in Equilibration" filed Jan. 13, 1981, supra, the dynamic range of the system was improved by the invention in U.S. application entitled "Flowmeter System with Improved Dynamic Range" by R. S. Loveland, supra, the gain of the flowmeter system was improved by the invention in the U.S. Patent entitled "Flowmeter System with Improved Loop Gain", supra, and a digital phase shifter and a means for calibrating the system was introduced into the system by the invention in the U.S. Patent entitled "Flowmeter System with Digital Phase Shifter and Calibration", supra.
Too, certain models of the transducer, itself, have been imoroved for use in high humidity environments, which are quite hostile to the proper operation of the system, such as in an intensive care unit (ICU) of a hospital, by the invention in the U.S. application of Bruce Mount and Con Rader entitled "Ultrasonic Airflow Transducer for High Humidity Environments", supra.
It was in the hospital use that the flowmeter system, whether or not incorporating any or all of the above improvements to the Hall and Loveland circuitry, was found to be needing still further improvement. In the use of the flowmeter system in the operating room for measurement of anesthetic gas flow, the variation in the density of the anesthetic gases required to be monitored simply could not be handled by the existing flowmeter systems. For example, the Hall and Loveland flowmeter, with certain of the above mentioned improvements, has proven to be satisfactory for gases of limited density range (typically, 10% CO.sub.2, balance air, to air at 35.degree. C.) but with gases of higher density, such as those required to be monitored for anesthetization purposes typically 3% halothane, 47% N.sub.2 O, and 50% O.sub.2, (considered equivalent in density to 100% N.sub.2 O), or with gases of lower density such as air at 40.degree. C., or 10% He balance O.sub.2, the flow measurement is not satisfactory. The reason for the unsatisfactory operation is that the phase-locked loop is unable to maintain a stable, locked condition for gases of density greater than 10% CO.sub.2, balance air, or of density less than air at 35.degree. C. due to a change in the transducer resonant characteristics. As a result, the system would go to an out-of-lock mode in the extreme density (higher or lower) ranges. The cause of the change in transducer resonant characteristics with increased density is not fully understood but the effect is that the response characteristic of the transducer undergoes a phase response change as well as an amplitude response change as a function of gas density. For example, the optimum operating phase for high-density anesthetic gas changes by up to +180.degree. from the optimum operating phase in air and, conversely, at low densities, the operating phase changes in the negative direction from the optimum in air, while the amplitude response peaks at about 10% CO.sub.2 balance air, and decreases for gases of higher or lower density.
To better understand the need for this invention, attention is directed to FIG. 1 which is a chart with the electrical output amplitude (db attenuation) and phase-shift (degrees) plotted against frequency (KHz). This chart was generated by actual tests on the various gases used in anesthesiology, when it was found that the present circuit was incapable of handling such gases. In this chart, it can be seen that the resonant frequency of the flow transducer in air at 25.degree. C. is approximately 39.5 KHz and the resonant frequency in air at 40.degree. C. is approximately 40.5 KHz. On the other hand, a gas which is 10% CO.sub.2 balance air, causes the transducer to resonate at a frequency of about 37.3 KHz, a gas which is 50% O.sub.2 50% N.sub.2 O produces a resonant frequency of approximately 34.3 KHz, and a typical anesthetic gas that is equivalent in density to 100% N.sub.2 O (3% halothane, 47% N.sub.2 O and 50% O.sub.2) produces a resonant frequency of 32 KHz. Also, from the chart it can be seen that, if the system were calibrated in air at 25.degree. C. at its resonant frequency (which is required, since for use in anesthesiology air must be measured as well as other gases), the phase-locked loop would be referenced to 0.degree. of phase-shift. Note that the output amplitudes of the transducer, when it is operating with the other gases, varies from a high amplitude with the gas which is 10% CO.sub.2 balance air, to a very low amplitude with the gas which is 100% N.sub.2 O but, more importantly, using 0.degree. as the optimum operating phase in air at 25.degree. C., the optimum operating phase in air at 40.degree. C. would be approximately -30.degree. , whereas, the optimum operating phases for the other three gases would be +90.degree., +130.degree. and +170.degree., respectively. Since a normal phase-locked loop will try to maintain operation at the reference phase (in this case, 0.degree.), it can be seen that the operating frequency of the transducer will be forced away from the ideal operating frequency in all other gases but air at 25.degree. C.
This affects the capability of the system to monitor the flow of the fluids by limiting the operating range of the system (referenced to air at 25.degree. C.) from a gas density equivalent to 10% CO.sub.2, balance air, to air at approximately 35.degree. C. Note, also, the phase-versus-frequency variation, at resonance of the transducer operating with various gases, is nonlinear, so a linear phase-shift compensation means added to the system would not suffice for flow measurement of all the gases under consideration. It is also apparent from FIG. 1, that the amplitude-versus-frequency variation at resonance of the transducer, operating with various gases, is nonlinear, so a nonlinear amplitude compensation means must be added to the system.
Accordingly, an object of this invention is to improve an acoustical wave flowmeter to enable operation with a wider range of gas densities than possible with the existing art acoustical wave flowmeters.
A more specific object of this invention is to provide an acoustical wave flowmeter so that the operating phase of the phase-locked loop will vary in a continuous manner according to a previously determined nonlinear relationship of phase-versus-operating frequency and so that the loop acquisition frequency of the phase-locked loop will vary in a continuous manner according to a previously determined nonlinear relationship of amplitude-versus-resonant frequency of the transducer in order to maintain a transducer at its optimum operating characteristics.
Still another and more specific object of this invention is to provide a flowmeter capable of monitoring gases used in anesthesiology.